This invention relates to the extrusion of foamed polymer compositions and more particularly to the extrusion of foamed polystyrene of improved characteristics.
In the manufacture of polystyrene foam the method wherein heated and pressurized polystyrene resin is mixed with an appropriate foaming agent and then extruded to a low pressure zone is well-known and widely used. Eventually the foam stock is subjected to a thermoforming step in the manufacture of articles such as foam cups, meat trays, snack food containers, etc. A variety of organic blowing agents have a solvating effect on polystyrene, which effect has both beneficial and adverse aspects. Blowing agents such as the lower molecular weight hydrocarbons, e.g., propane, butane, pentane, etc., and chlorofluorocarbons have such a solvating effect on polystyrene. From the standpoint of placticizing the polystyrene during the melt mixing of the same, this has a beneficial effect in that the melt can be more easily worked and mixed during the stage or stages prior to extrusion thereof. If the solvating effect is too great, deficiencies are encountered during subsequent thermoforming of the polystyrene foam. This is due to the strong affinity of such blowing agents to the polystyrene. Thus, after the foam is extruded, these blowing agents tend to remain in the cell walls of the polystyrene and, even after aging for up to several days, the polystyrene foam will tend to sag or distort prior to or during thermoforming. Therefore, the foam material produced by the above method must be aged carefully to permit evaporation of the blowing agent down to a specific level so that sagging and warping during thermoforming is avoided.
It is an object of the invention to provide a process which permits better control of residual plasticizing type blowing agent remaining in the foamed polystyrene after extrusion thereof.
It is another object of the present invention to provide a process which yields a foamed polymer of comparatively lower density.
It is still another object of the present invention to provide a process which minimizes or eliminates the need for aging foamed polystyrene prior to thermoforming.